A printed wiring board, which is a structure containing a conductor circuit, is a printed wiring board in which a plurality of wiring layers are formed on a core board, and includes a copper-clad laminate which is the core board, an interlayer insulating material provided between the respective wiring layers, and a solder-resist provided on the topmost surface. Semiconductor elements are usually mounted on the printed wiring board through a die bonding material or an underfill material. Further, they may be entirely sealed by a transfer sealant, or a metal cap (cover) for improving heat dissipation may be mounted, as necessary. In recent years, reductions in size and weight of semiconductor devices have continuously progressed, and densification of semiconductor elements and multilayer printed wiring boards has also progressed. Further, a mounting form such as package on package in which a semiconductor device is stacked on a semiconductor device is also widely implemented, and a mounting density of the semiconductor device is expected to increase more in the future.
Incidentally, it is necessary for vias (openings) for electrically connecting upper and lower wiring layers to be provided in an interlayer insulating material of a printed wiring board. If the number of pins of a flip chip mounted on the printed wiring board increases, it is necessary to provide openings corresponding to the number of pins. However, since a conventional printed wiring board has low mounting density and has a design in which a mounted semiconductor element has from several thousand pins to about ten thousand pins, it is not necessary to provide openings with a small diameter and a small pitch.
However, as miniaturization of the semiconductor element progresses and the number of pins increases from tens of thousands to hundreds of thousands, there is an increasing need for reduction in sizes of openings formed in the interlayer insulating material of the printed wiring board according to the number of pins of the semiconductor element. In recent years, a printed wiring board in which openings are provided by a laser has been developed using a heat-curable resin material (e.g., see Patent Literatures 1 to 4).